I think I do not understand this challenge. From the text:
A universal cycle is a cycle where all sets of 3 different values appear exactly once in consecutive neighbors.
It follows that I should "make" each possible set of three values exactly once. Checking the existing sets (before I fill the gaps), I see that for example the sets:
are not present. The only way to make one of the above sets is to write it in the three consecutive blanks. Once I select one of them, there is no way to "make" the other. What am I missing?
There is no 9.
Thanks for the reply, my mistake for not spotting that.